Iraq slipped further into civil war today when gunmen hijacked 12 minibuses and kidnapped as many as 40 passengers outside Baghdad, while a car bomb left 15 people dead at a wedding party in the capital.

Between eight and 10 gunmen stopped the convoy of minibuses on the dangerous road heading north of Baghdad into mixed Sunni and Shia farmlands which have seen bouts of prolonged bouts of ethnic bloodletting in recent weeks.

The buses had left the main road to avoid an American checkpoint, an Interior Ministry official said, when they ran into a guerrilla roadblock at Tarmiya, a well-known insurgent area.

Most of the passengers marched off to face probable execution were Shias heading to Balad, a town to the north which Shia militiamen purged of Sunnis two weeks ago in revenge for the beheading of 17 local Shia men.

Almost 100 people were murdered before US troops moved into the town, and Sunni militants vowed to avenge the dead.

As they executed their vengeance, only the driver of one of the buses managed to escape.

In Baghdad, up to 15 people, including four children, died when a car bomb exploded as groom’s family pulled up to a wedding party in the Shia district of Ur. Seven more people were wounded.

"A terrorist left a car parked on the street corner and, when the bridegroom’s family arrived in a convoy, he set off a bomb," an Interior Ministry official said. "The groom survived but many more died."

The neighbourhood is close to the Shia area of Sadr City, where 33 people died the day before in another car bombing.

That attack was blamed on Sunni extremists trying to stoke further sectarian conflict by attacking the stronghold of the Mahdi Army, the powerful militia of the Shia cleric Hojatoleslam Moqtada al Sadr.

The rebellious preacher won a political victory yesterday when Nouri al-Maliki, the Shia prime minister, ordered US and Iraqis forced to lift their blockade of Sadr City, which had been sealed off in the hunt for a missing Iraqi-American soldier.

The soldier was kidnapped after slipping out of his base and meeting with a woman he had secretly married in the capital, relatives said.

The Mahdi Army had threatened unspecified retaliatory action if the blockade was not lifted, as well as a city-wide strike, if the week-long stranglehold of checkpoints and car searches was not lifted.

As the coalition forces pulled out, Sadr City youths celebrated by waving banners from speeding trucks and mopeds.

"Your patience and unity brought victory," said a triumphant statement from Hojatoleslam al-Sadr’s office. Mr al-Maliki has promised to rein in militias such as the Mahdi Army, but has so far proved incapable of doing so without the collapse of his coalition government, consisting of parties with powerful armed wings.

The violence came a day after Dick Cheney drew links between the recent increase in attacks in the war-torn nation and the forthcoming midterm elections in the US.

The Vice President said that insurgents were keeping track of US opinion via the Internet and had stepped up their assaults in order to influence the midterm vote, which will take place next week.

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